High voltage direct current (HVDC) power systems comprise protection and control systems arranged to protect, monitor and control the functioning of devices forming part of the power system. The protection systems prevent, among other things, short-circuits, over-currents and over-voltages in e.g. power transmission lines of the HVDC system.
Protective relays are used throughout the HVDC system for providing such protection and control. The protective relays detect and isolate faults on transmission and distribution lines by opening and closing circuit breakers. It is not always necessary to perform a complete interruption; instead a commutation to an alternative path is performed. In essence the current in part(s) of the original current path will stop flowing, but it will not be interrupted, only redirected. To achieve this, a HVDC breaker is used.
FIG. 1 illustrates schematically a basic conventional direct current (DC) circuit breaker, also called DC breaker, which is arranged along a DC line L carrying a direct current I. The DC breaker 1 is designed so as to be able to break or commutate the direct current I. To this end the DC breaker 1 comprises an alternating current circuit breaker 2, denoted interrupter in the following, connected in parallel with a resonant LC branch 3, 4, i.e. a capacitor 3 connected in series with an inductor 4. A non-linear resistor 5 is connected in parallel with the LC branch 3, 4 for limiting the capacitor voltage when the direct current I flows through the capacitor instead of through the interrupter 2. The inductor 4 may, but needs not to, be a physical component; the leakage inductance in the circuit can often be enough.
In the following, a usual interrupting process is described. Upon interrupting or breaking the direct current I, a current is carried between the contacts of the interrupter 2 through an arc, and this arc current Iarc has to be extinguished. FIG. 2 illustrates the arc characteristics of the arc current Iarc in the interrupter 2. For interrupter currents Iarc up to approximately 5 kA the arc voltage/current slope is negative, which causes a growing oscillation against the LC branch 3, 4. When the oscillating current has grown enough, i.e. so as to be equal to the direct current I, the arc current Iarc reaches a current zero crossing, whereupon the arc is extinguished and the total direct current goes through the capacitor 3. The voltage of the capacitor 3 then grows rapidly until it reaches the knee point of the non-linear resistor 5, e.g. a surge arrester, which is arranged to limit the voltage on the capacitor 3. The capacitor voltage constitutes a counter-voltage in the circuit causing the current I to decrease until it ceases.
The above-described conventional DC circuit breaker 1 functions properly for transmission line or HVDC circuit direct currents I up to approximately 4-5 kA. For higher currents, there are two main limiting factors in the interrupting process just described:                The steady state current capability of the interrupter is today limited to approximately 5 kA.        The arc characteristic, as shown in FIG. 2, is a curve, which beyond a certain arc current Iarc loses its negative slope and becomes flat, which makes it difficult to have an oscillation large enough to cause a zero crossing in the arc current Iarc. The corresponding direct current I at which the characteristic becomes flat is not an exact point but is somewhere around 4 to 5 kA.        